It sounds like you have a CSS length. Rust doesn't have any types for CSS in the standard library, so you'll have to tell us what library you are already using for working with CSS. What is the context in which border-radius: 10px; is appearing? Show much more of your code.
Here's the rust slint documentation for the type mappings:
There they say both logical and physical lengths are mapped as bare f32, that is, 32 bit floating point. You should also not use a px suffix on the value outside of slint!, native Rust doesn't support custom suffixes.
You can't use rust code (so rust contants) directly in the Slint DSL.
I think the way to do what you're doing is with a Slint global property. Something like this: